Various implant-related devices and systems have been created in recent years in an effort to anchor dental prostheses more directly and flexibly in the mandible or maxilla than was possible using conventional dentures or bridges. A scientific breakthrough in the area of dental implants was achieved by Swedish Professor Per-Ingvar Branemark and coworkers. Combining a two-stage surgical technique with the use of titanium fixtures, these scientists achieved predictable results in surgical placement of permanent dental implants. Their continued studies in the early 1960's provided the basis of modern implantology. Such dental implant systems typically employ an anchor fixture or "implant" which has been inserted into the bone and from which extends prosthesis-supporting structure typically coupled to the implant using a fixation screw or other desired fastener. FIGS. 1, 2 and 3 illustrate a typical Branemark implant system that is described in the legends accompanying FIGS. 2 and 3, and that is currently commercially available.
In the three decades that have elapsed since the pioneer Branemark work, extensive research and development effort has gone into efforts to improve and perfect such endosseous implants. Today such implants are basically of three different types: blades, screws and cylinders. Screws and cylinders both require that a hole be drilled into jawbone. In the case of screws, the hole is then either tapped to accept a threaded screw implant or a self-tapping screw implant is threaded thereinto. This latter type is illustrated in FIGS. 1, 2 and 3. A cylinder-type implant having a threaded central bore is installed by first drilling a hole into the bone. The cylinder implant is then introduced into the hole and forcibly tapped into place with a small mallet. With both screw and cylinder type implants, a cover or sealing cap screw is threaded into the open bore of the cylinder to protect and seal the top of the cylinder following first stage surgery implantation. At a later stage of the implantation installation, an abutment, which is typically a hollow cylinder with a central bore, is placed on top of the cylinder through the gum tissue located above the bone to provide a platform for a replacement tooth or artificial dental appliance. An abutment screw extending through the abutment into the implant threads, and a gold prosthetic retaining screw extending through the prosthetic device and into the upper end of the abutment screw, are respectively used to secure the abutment and prosthetic device in place in the implant component assembly.
Thus, as stated in "Osseointegration in Dentistry An Introduction" by Worthington, Lang and LaVelle, published by Quintessence Books, copyright.sub.------, at page 37:
"There is an apparent lack of consensus among researchers regarding the basic design of an implant, as evidenced by the multiplicity of implant designs. Some state that it is critical to load the bone in compression while minimizing the shear forces; the screw-type implants reflect this theory. On the other hand, proponents of cylindrical designs stress the importance of the implant deriving its support through shear forces applied to the implant-to-bone interface; the cylindrical press-fit implants reflect this theory." PA1 "For those clinicians who are placing implant assembly systems, the second worst feeling next to a loosening implant is that of a loosening abutment--one that has loosened because the fixation screw holding it in place has broken or come loose." PA1 "More often than not, when screws break, they break off at the thread which leaves them submerged in the implant. This makes them very difficult to remove without damaging the implant. The inventor has seen many interesting techniques for retrieving broken screws, but all it takes is to damage a single thread of just one implant and potentially an entire case can be put in jeopardy. PA1 "It can be argued that the only reason these screws break is because of poor treatment, planning and excessive occlusal overloading of the implant assembly. However, the inventor has seen numerous cases of screw breakage--everything from the screw-retained single tooth, to a screw retained dolder bar supported by six implants." PA1 "One noticeable feature among the problems and complications encountered during the Toronto study was the large number of gold screws that were reported to have fractured. Of 274 implants that were placed to support 49 protheses, 53 fractures of the gold alloy screws were noted, 14 framework fractures and 9 abutment screw fractures. Similar observation emerged from a replication study conducted at the University of the Witwatersrand in which Shakelton et al. reached the conclusion that more than 50% of prosthetic problems are related to stress factors acting on the prostheses . . . To have screws fracturing as a normal event would not be conducive to sustained levels of patient confidence." PA1 "At an implant symposium (September 1993) in New York USA when the topic of discussion was confined to implant failures, Dr. Carl Misch was quoted as saying, "Screws rattle loose." (Indirect Verbal Communication.) This is true, but not only do they come loose, they also fracture. This fracturing, which has been reported repeatedly in the literature, is due to biomechanical overload which frequently causes the weakest link in the system to break. In the inventor's opinion, to have screws fracturing as a normal event is no way to maintain patient confidence or deal with the problem of biomechanical overload." PA1 "The conditions most frequently seen during periodical recall appointments which require attention are gingivitis, soft tissue hyperplasia, small fistulae, exposed implant threads, fracture of the abutment screw, fracture of the anchorage mechanism between the prosthesis and the abutment, loss of optimal occlusal contacts, and fracture of the prosthodontic framework." PA1 "Furthermore, because of the excessive torque sometimes used to tighten the healing cap on the fixture, breakage of the fixture and/or the healing cap can occur." PA1 "The most common prosthesis problem that the author has thus far noted is the loosening or breakage of the gold locking screw with the resultant loosening of the prosthesis."
For further background on the history and relatively current state of the art, see the extensive discussion in the Willoughby U.S. Pat. No. 5,527,182 issued Jun. 18, 1996 and incorporated herein by reference.
However, regardless of the type of dental implant design currently being made commercially available, almost all such systems employ some type, and usually a plurality of different types, of fixation screws to retain the implant components in assembly in order to permit removal, repair, restoration, etc., of the implant components as well as to retain the cover screw or healing cap during the first stage surgery, followed by the second stage surgery and abutment connection. Although cementable retained prostheses are an alternative to screw retained prostheses, nevertheless screw-retained abutments are most popular due to their many advantages and represent the most popular type system offered by almost every prosthetic dental implant company. Nevertheless screw-retained systems present long standing and well recognized problems of screw breakage and fracture. To quote the Willoughby '182 patent at column 27, lines 6-10:
Continuing at lines 25-38:
In addition, at column 58, lines 4-14, the Willoughby '182 patent quotes an article by Monteith regarding the results of the University of Toronto's 1990 longitudinal implant study:
As further stated at column 67, lines 59-67 and continuing at lines 1 and 2, column 7, of the '182 patent:
Further recognition of the screw breakage problem is set forth in the aforementioned Worthington et al textbook, Osseointegration in Dentistry, at page 39, FIG. 4-2, illustrating an implant complex with a fractured prosthetic retaining screw, caused by exceeding the mechanical limits of the screw. Further, at page 121 of this work, dealing with the status of the implant prosthesis at the recall appointment, the text states:
The breakage problem as it relates to installing and removing the healing cap or cover screw that is placed in the interval between surgery and abutment selection/prosthesis fabrication is also recognized in the Wilson et al U.S. Pat. No. 5,336,090 which points out at column 2, lines 66-68 and continuing in column 3, line 1:
To the same effect is the Salazar et al U.S. Pat. No. 5,362,234, in citing the observation of Desjardeins in a publication entitled "Dental Implant and Prosthodontics", J. B. Lippincott Company, 1991:
Additional patents presenting a sampling of those concerned with the problems of loosening or breakage of the fastening elements in the implant assembly components are:
______________________________________ Voiteik 5,106,300 Perry 5,108,288 Kyist 5,169,308 Milne 5,704,788 ______________________________________
Even in the ordinary every day experience of the automotive mechanic, machinist or tool and die set-up man, the breakage of a screw or a bolt, leaving the threaded leading end fragment of the bolt or screw shank threadably embedded and recessed down into the hole of the female threads in the receiving member, presents a very vexing problem. One can only imagine how this is compounded by orders of magnitude due to the difficulties of dealing with the miniaturized components of an implant assembly located in a difficult location of limited accessibility in the mouth of a patient. Typically, when a screw is fractured off inside the implant fixture, the clinician tries to cut a slot or hole in the bottom of the broken section of the screw so that it can be unscrewed from the fixture. However, to try and tease the broken piece out with a tip of a small instrument, such an explorer, scaler, etc., which is often the general method used today to retrieve these broken pieces, is to say the least a very difficult procedure for both the clinician and the patient. Moreover, such "fishing" efforts can result in the tip of such retrieval tools scarring the internal threads of the implant fixture as the tool tip is being inserted thereinto when trying to cut a hole in the broken section and/or while trying to tease out the broken piece. Damage to the internal threads of the implant fixture can, of course, jam and block the unscrewing of the broken off shank of the fastener, further compounding the problem, as well as ruining the fixture in situ.